Understanding of molecular mechanisms of aging is indispensable for the development of rationale strategies for therapeutic intervention in aging, which is the most potent risk factor for any age-related disorders including diabetes, heart attack, stroke, osteoporosis, cancer, and dementia. Recent studies have identified Klotho as an aging suppressor gene that extends life span when overexpressed and accelerates the development of multiple aging-like disorders when disrupted in mice. The long-term goal of this proposal is to understand the mechanism by which Klotho suppresses aging. The klotho gene encodes a single-pass transmembrane protein whose extracellular domain binds to multiple fibroblast growth factor (FGF) receptors and modifies their affinity to FGFs. Notably, Klotho enhances the activity of FGF23, a hormone that suppresses phosphate reabsorption and vitamin D activation in the kidney. In addition, ablation of vitamin D activity rescues many aging-like disorders in Klotho-deficient mice, implying a novel concept that the FGF signaling and vitamin D metabolism may participate in aging processes in mammals. The fact that the extracellular domain of Klotho is shed and secreted in the blood and urine has led to two hypotheses on Klotho function that are not mutually exclusive. First, Klotho may regulate phosphate and vitamin D metabolism through enhancing FGF23 activity. Second, the extracellular domain of Klotho may function as a phosphaturic factor. Specific aims to test these hypotheses are to: (1) Investigate the mechanism by which Klotho and FGF23 regulates vitamin D metabolism. Specifically, effect of Klotho on expression of key enzymes involved in vitamin D metabolism will be determined. (2) Test whether Klotho functions as a phosphaturic factor. Effect of the recombinant extracellular Klotho peptide on phosphate metabolism will be tested in cultured cells and mouse models. These studies will promote better understanding of Klotho protein function, which may eventually contribute to understanding of molecular mechanisms of aging.